Downhole combination tool

ABSTRACT

A bidirectional drilling downhole steering and survey tool for detecting ferromagnetic bodies from an off vertical borehole, transmitting the signals from the downhole sensors to surface equipment while drilling, and for calculation of the range and direction to the detected ferromagnetic body for the use of the directional driller in intercepting or avoiding the ferromagnetic body. In combination with the steering and range and direction to ferromagnetic body capabilities, the downhole tool may also include sensors for detecting formation bed interface, apparatus for electric or gamma ray or radioactive logging, neutron density sensor apparatus and other formation evaluation or correlation apparatus. The downhole tools may be packaged, in either an electric line transmitted tool of MWD transmitted tool.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a continuation application of copending application Ser. No.07/264,445 filed on Oct. 28, 1988, now the U.S. Pat. No. 5,064,006.

FIELD OF THE INVENTION

This invention relates to a new and improved oil well downholecombination tool for drilling, steering, directional surveying, magneticranging, and other types of formation measuring including and notlimited to radioactive, resistivity, and conductivity measuring. Thetool can be configured to utilize either electric line or mud pulsetelemetry for measurements while drilling, or for both or somecombination of both telemetry systems. Many configurations of thesteering, surveying, magnetic raging and measuring features of thecombination tool could be packaged for a particular job and tailored tothe field conditions.

This invention generally relates to the determination of a course of anoff-vertical borehole, and determination of range and direction to asubterranean target from a designated position or succession ofpositions in the off-vertical borehole. More particularly this inventionis directed to a system for acquisition of a wide variety of well andformation data in addition to data relating to borehole orientation andmagnetic body location which may be coupled with a telemetry system totransmit the data to the surface in real time or quasi-real time whiledrilling.

BACKGROUND OF THE INVENTION

The invention involves measurement, while drilling, of the earth'smagnetic and gravity fields as a reference frame to determine theorientation of a tool in the borehole. Also, the tool senses anomaliesin the static magnetic field caused by a ferromagnetic target body orsenses a time varying magnetic field created by the injection of anelectrical current at some remote point or in the vicinity of a targetbody. The measurements of the anomalies are used to determine directionand distance, or range, from the position of the tool in theoff-vertical borehole to the target. The tool of this invention is alsoable to provide data that enables simultaneous steering andferromagnetic detection for controlling the trajectory of the boreholeas may be desired, although the steering tools in a wirelineconfiguration will not be used while drilling with a rotating drillstring. In this case the drilling rotation would be stopped momentarilywhile the steering and ranging information or survey was done.

Where the drillstring is not rotating but instead the drill bit isturned by a mud motor at the bottom of the hole, the tool may be used ineither a "steering tool" configuration with wireline telemetry to andfrom the surface, or in the "MWD" measurement while drillingconfiguration such as that using mud pressure pulse telemetry to andfrom the surface. All of the steering information would be availablewithout the need for momentarily stopping the drilling to take theranging survey. The apparatus also includes a sensor or sensors forcorrelating formation bed interface or measuring geologic strataencountered, apparatus for electric or gamma ray, or radioactivelogging, and a telemetry system for transmitting the data to the surfacefor collection and use, and for transmitting control instructions tosense or downhole microprocessors from the surface to the tool. Thecombination of sensors for formation evaluation with the magnetic andgravitational reference frames using magnetometers and accelerometersimproves the quality of the information available to the explorers ofthe reservoir during downhole continuous drilling operations. Themagnetometer and accelerometer arrangement can be used both for steeringthe downhole tool and, in conjunction with the formation evaluationequipment, for directional borehole surveying or for correlation withoffset wellbores.

The drilling system involved in drilling an off-vertical well typicallyincludes a turbine driver, or "mud motor", and a rotating bit which isconnected to a non-rotating drillstring by means of a short, slightlybent section of drill pipe, or "bent sub", an articulated assembly, or ajetting assembly. Alternatively the housing of the turbine may be bentso as to cause the rotating axis of the bit to be slightly out ofalignment with the borehole. The orientation of this system may bealtered under surface control to advance the borehole along anaccurately defined course for interception or avoidance of a target.When drilling such an off-vertical well, it is often necessary to changeits course, or path, by controlling the position of the drill itself.The directional drilling processes require a precise knowledge of theorientation of the drill and its path so that the course may beevaluated and decisions made concerning alterations of the boreholepath.

A device, known in the art as a steering tool, provides suchmeasurements of the orientation of the drill in the borehole withrespect to a reference system while the drilling operation is underway.The steering device usually includes component or directional typemagnetometers and accelerometers for sensing the earth's magnetic andgravity fields, thus to provide the required reference framework. Thesteering device is secured with respect to the drill with a key or othermeans nd thus fixes the magnetic and gravity reference frame to thedrill itself.

The device is mounted in a non-magnetic region sufficiently removed fromthe drill and other ferromagnetic components in order to respond only tothe magnetic fields external to the drill system. As the drillingproceeds, the device measures the components of the magnetic and gravityfields and telemeters the measurement signals to the surface. At thesurface they are resolved by suitable computation to provide the drillerwith values of the orientation of the drill system and/or trajectory ofthe wellbore.

A representative steering tool is described in U.S. Pat. No. 3,791,043.This steering tool consists of a triaxial component magnetometer tomeasure the earth's magnetic field and two bubble inclinometers tomeasure the gravity field. Surface instrumentation resolves the magneticand gravity measurements to provide readings of the azimuth andinclination of the drill to the driller as well as orientation of thebent sub with respect to the high side (or vertically topmost point) ofthe hole (toolface). The drill orientation is changed as appropriate inaccordance with the readings to adjust the course of the borehole. ThisU.S. Pat. No. 3,791,043 is specifically incorporated herein byreference.

It is sometimes necessary to determine the direction and range to anearby subterranean ferromagnetic body from a position in the boreholebeing drilled. For example, adjacent preexisting wells must be sensedand avoided when a borehole is being drilled as when multiple wells arebeing drilled from a common small spaced platform. The accuracy of thesensors of the magnetic ranging system, in combination with a telemetrysystems that makes readings available to the surface operators whiledrilling is in progress, is especially suited for avoiding existingwells. Yet another example would be the search for subterranean orebodies exhibiting magnetism, or those which do not exhibitferromagnetism. This combination tool will be useful for drilling apilot shaft to search for conductance properties in applications such asgold for silver mining. Another application could reveal large coalstreaks by the nonconductance or resistance encountered. Another exampleis when the borehole being drilled is an off-vertical relief well beingdrilled to intercept a blowout well at a depth below the disturbancecaused by the blowout. U.S. Pat. No. 4,072,200 is directed to thisapplication and includes magnetometers which detect remnant or inducedstatic magnetic field anomalies resident in the ferromagnetic bodyinvolved in the second well or body to be located, or alternatelymagnetometers which detect a time varying magnetic field created in thewell or target body by an electrical current injecting system controlledby the operator. As appropriate, parts of the disclosure of U.S. Pat.No. 4,072,200 are included as part of the disclosure of thisapplication. All of U.S. Pat. No. 4,072,200 is specifically incorporatedin this application by reference.

The techniques of drilling one well to intercept another with a targetwell exhibiting a signal is taught in U.S. Pat. Nos. 3,285,350 and3,731,752.

Given the case where the target is inaccessible, as in a blowoutsituation, a method for an approach to the target location is disclosedin U.S. Pat. No. 3,725,777, in which the total magnetic field andmagnetic compass readings were made and compared through least squaresfit analysis to various assumed positions and magnetizations of thetarget well casing. However, this technique is subject to locationambiguities.

The measurement of magnetic vector components, as taught in U.S. Pat.No. 4,072,200, allows the determination of target polarity as well asdirection and resolves such location ambiguities. In addition, themethod of determining range from the tool to the target, using gradientmeasurements, is applicable for both the static and time varying fieldtechniques.

As shown in the prior art, there have been provided separate tools forsteering drilling, and for borehole orientation; anomaly seekingmagnetic tools; for either residual and/or time varying magnetic fieldtools, and formation evaluation tools. Each such tool requires aseparate survey to be run with the resulting time and complexityinvolved. Each operation involves stopping the drilling operation,removing the drillstring with the steering tool or MWD, running a surveywith the ferromagnetic body seeking system, returning the drillstring tothe well bore, and then continuing the drilling operation. The procedureis expensive, time consuming, complex, and subject to introduction oferror. In addition, the bore hole erodes with time.

The combination of the steering tool with the ferromagnetic body seekingsystem, or magnetic ranging components, into a single tool that operateswhile drilling and which may include a range of formation evaluation orcorrelation sensors is a significant advance over the equipmentpresently available in the industry. Some advantages of the combinationare the enhanced orientation capabilities of the ferromagnetic sensingsystems, the cost and time savings of the continuous drilling andsurveying compared to trips out of the hole for running separatesurveys, and the improved quality of the formation evaluation whichresults from the more pristine borehole condition as measuredimmediately after drilling in contrast to measurements taken after adrillstring is tripped out when caving, sloughing, and invasion maydistort the evaluations. Drilling fluids can be highly caustic, and tendto react With shales, leading to sloughing of formation materials andtool sticking.

Additional advances in safety over the prior tools are especiallyneeded, and are provided by the present invention, in the circumstancesof drilling to intercept a blowout well. Any open hole survey operationentails a certain degree of risk, however, hydrostatic conditionsencountered in relief well drilling are often abnormal and irregular,and consequently the potential for blowout is higher, The combinationtool is either on electric line or a MWD configuration eliminates theneed to come out of the borehole with the drillstring so that surveydata may be obtained, thus reducing the likelihood of another blowout.Since the drillstring remains in-hole and fluid circulation continues,it is easier to balance or control the hydrostatic pressures,eliminating a common cause of blowouts,

Further, advantages to the combination tool are capabilities not foundpresently in the industry. The magnetic ranging orientation system has aresolution allowing directional drilling accuracy. All directionaldrilling operations require some method of orienting the downhole tooland the high quality, magnetic sensors in the combination tool provideaccurate information for orienting the tool. In combination with adirectional drilling assembly the magnetic ranging system will operatein a borehole through unusually high inclinations through which it wouldbe impossible to run a wireline survey at borehole inclinations near thelimit of operability for wireline survey, formation evaluationinformation in real time is very valuable, because it provides a recordof information about the formation even if the well blows out, or thetool is stuck, or the hole is lost for any other reason. Even when thedrilling assembly is lost, the components of this composite surveysystem can be retrieved for reuse, or at least the survey data can beretrieved or retained.

Further important improvements over present practice are the pinpointingof responsibility for the drilling and survey operations and a reductionin personnel and therefore a reduction in costs resulting from thecombination of steering, magnetic ranging and formation evaluation intoa single system operated by a single crew. A very significant advantagealso results from transmitting the magnetic ranging information whiledrilling rather than obtaining it in intervals as is furnished bypresent wireline tools. Drilling must be stopped to run the wirelinetools, so the tendency is to overdrill before stopping to evaluate theformation and determine the trajectory of the borehole. Because thedrilling has stopped and the entire rig is idle during the wirelinesurvey and following period when the data is analyzed, there is pressureon the person or persons doing calculations for determining targetposition based upon large quantities of data taken over long intervalsof drilling and corresponding to a substantial length of the borehole.Consequently, mistakes in calculations are more likely. With continuousdata transmission occurring during drilling operations, the progress andtrajectory can be monitored and the calculations made much moreefficiently without the mistake causing pressure posed by an expensiveidle drilling rig.

It is highly desirable and advantageous to be able to combine theoperations which provide steering information with measurementsconcerning the range and direction to a subterranean body as well aswellbore trajectories and formation evaluation. The present inventionprovides this desirable combination by including the static magnetic,gravity, and time varying magnetic measurements in a drilling system.The invention also provides the capability for accomplishing other welland formation measurements including electric or gamma logging,formation temperature, etc., during the drilling operations. Thisfeature provides drilling personnel with the capabilility forimmediately and continuously receiving well and formation data to ensurethe safety and efficiency of the drilling operation. This information isavailable to be used in combination with formation evaluation deviceswhich require knowledge of the earth's magnetic and/or gravity fieldsfor orientation. The information measured is converted to a singleserial composite signal and transmitted from the subsurface tool to theearth's surface by suitable telemetry means such as by hydraulic signalpulses in the drilling fluid column, by a single electrical conductor,solid state memory with periodic readout, or the like. With the toolfixed into the drilling system, the invention also provides thenecessary information to serve the drilling steering purpose.

In the past, acquisition of well survey signals at the surface wasdependent on the number of signal electrical conductors extending fromthe downhole sensing tools in the drilling zone, to the surface. Forexample, an electrical signal transmission cable having seven conductorshas been widely used, thus limiting the acquisition to seven signals.The present invention incorporates signal processing circuitry in thedownhole tool which receives and measures any number of selected signalsrelating to conditions of the well ;bore and formation. These signalsare received as analog signals and the circuitry of the down hole tooldigitizes and multiplexes them and processes the signals by way of amicroprocessor to form a single composite signal. This single signal canthen be transferred to surface signal processing and display equipmentby a variety of signal transmission systems such as single electricalconductor, by mud pulse fluid signal transmission, by retention in solidstate memory for later retrieval, or by any other suitable means.

OBJECTS OF THE INVENTION

A principal object of this invention is to provide a single tool formany combinations of drilling steering, directional surveying, magneticranging, formation evaluation logging and measurement of dynamicmechanical properties of the drillstring. This combination tool could beprovided with either a mud pulse telemetry system in an MWDconfiguration or an electric line telemetry system or both. The toolalso could be provided with downhole memory storage to collect datawhich, would then be read out either by running an electric line down toa wet connection terminal, or by retrieving the tool, including memorysection for reading at the surface. This would provide a quasi-real timedata read out.

Another object of this invention is to provide means to sense componentsof the earth's magnetic and gravity fields and also to sense smallstatic magnetic, and small time varying magnetic fields, and/or inducedelectrical and magnetic fields emanating from a second well and toprovide means to relay the information sensed to the surface whiledrilling is in progress, and to provide a means fixed to a drill systemwhereby the direction of boring may be controlled from the surface.

Another object is to provide a means of injecting an electrical currentso that it may be conducted by the second well and subsequently cause atime varying magnetic field concentric to the well to be developed whichwould have, a fall off rate of 1/R, the radial distance from the targetwell.

Another object of the present invention is to make measurements at aplurality of locations within the well to determine: the earth'smagnetic field for reference orientation; earth's gravity field forreference orientation; static magnetic field anomalies associated withthe target well for use in determining distance by gradient ranging anddirection by vector resolution; and time varying magnetic fieldmeasurements associated with the target well for use in determiningdistance by gradient ranging and direction by vector resolution.

Another object of the present invention is to provide a means to acquireformation evaluation and correlation information such as the reservoircharacteristics, porosity, permeability, neutron, density, andtemperature of the formation in conjunction with the orientationproduced.

Another object of the present invention is to provide a suitable surfaceapparatus to provide azimuth, inclination, and toolface orientation ofthe drilling system and to provide values of the quantities measured tobe used to calculate direction and range to the target well.

An even further object of the apparatus is to provide suitable telemetryfor the downhole signal to enable the presentation at surface receivingequipment of signals identifying the various parameters as measured inthe borehole while drilling, thereby negating time degradation of thehole. and to present at the downhole tool instructions controlling thedownhole sensors and microprocessors.

SUMMARY OF THE PRESENT INVENTION

According to the present invention, the re is provided the capability ofsurveying subterranean magnetic bodies of material from an adjacentoff-vertical borehole, and at the same time surveying the boreholeitself to determine its azimuth and inclination, that is, theorientation in space of the borehole being drilled, in combination witha range of formation evaluation measurements. The surveys may becombined to continuously provide information concerning the location inboth distance and direction of the target subterranean body from theborehole and the borehole and toolface orientation measurements forsteering purposes and so that the information from the formationevaluation sensors can be most accurately tied to a specific pointwithin the reservoir.

In one aspect of the invention, a magnetic survey of a target body ismade which measures the components of the total magnetic field, whichfield includes the target magnetic field superimposed upon the earth'smagnetic field. The components of the magnetic fields are measuredorthogonal to one another and along the axis of the borehole at thepredetermined separation within the tool. Additionally the earth'sgravity field is measured and using the measurements, both theorientation of the borehole and data used to determine distance anddirection from the borehole to the target are telemetered to thesurface, calculated and displayed continuously, while drilling is inprogress. In another aspect of the invention the magnetic survey is ofthe static magnetic field exhibited by the target.

In another aspect, a time varying magnetic field of a predeterminedfrequency is created in the target and the time varying magnetic fieldis surveyed.

To accomplish the surveys a system is provided having a downhole toolwith sensors for sensing the magnetic and gravity fields and withsensors for sensing the other formation characteristics. The toolgenerates individual signals indicative of the measurements which arethen digitized and multiplexed into a single serialized signal which canbe transmitted from the sensing apparatus to the surface where it isdecoded and separated into the individual signals and the signals usedto compute and display the desired steering information and to displayor record the radioactive, conductive, resistive, or other desiredformation properties. The system may also be provided with a signalgenerating apparatus at the surface for telemetry of controlinstructions down to the tool, which is provided with receivingapparatus to receive and interpret the signals from the surface. Byproviding the ability to send signals from the surface down to the tool,the timing, and content of the downhole survey information, could bevaried as desired. For example, it might be desired to receive moredetailed formation evaluation information rather than updating thesteering information as often as would be usually desired. In this casea signal could be sent down to the tool to cause steering information tobe sent every foot instead of every three inches, and instead of sendingthe steering information the formation survey information could bemeasured in greater detail at more frequent intervals and telemetered tothe surface. Once the formation of interest had been adequatelyinvestigated, a signal could be sent from the surface down to the toolinstructing the tool to return to its more usual mode of operation withthe more frequent steering signals being telemetered to the surface andless frequent or less voluminous formation evaluation information beingtransmitted to the surface.

The downhole tool comprises a non-magnetic housing for the sensors whichmay be fitted in combination into the housing. The tool configured forelectric wireline telemetry is constructed to be removably fitted intothe interior of a drill collar, in contrast to the tool configured formud pulse telemetry or MWD which is generally configured as being builtin to a non-magnetic sub, or drill collar, although it is possible andin some cases may be desirable to removably fit the downhole tool intoan MWD configuration. In an electric wireline configuration the tool aswill be described in more detail below, is lowered down into theinterior of the drillstring and fitted into a connection within thedrill collar. The housing is integrated into the drillstring in anon-magnetic MWD collar. One common example would be a monel collar. Inan electric wireline tool system, the tool is fitted so that in theevent of the drilling string sticking, the tool can be retrieved by theelectric wireline for use in another housing. In a mud pulsetransmission MWD system, downhole memory could be provided with a meansfor running down a wireline to connect into the system and retrieve thetool housing or at least the data to retain drilling information in theevent that the drillstring sticks.

The tool, either electric line or MWD conveyed, contains a pair oftriaxial component magnetometers of the fluxgate type which measures thestatic magnetic field having their individual sensor axes alignedparallel to one another and spaced apart along the longitudinal axis ofthe tools by separation of approximately one meter. A time varyingmagnetic field is measured by two or three coil type variometers alignedin the tool in the same fashion as the magnetometers. Gravity fieldmeasurement is provided by three accelerometers mounted in the housingperpendicular to one another. The axes of these accelerometers arealigned parallel to the magnetometer axes described above. Thecombination downhole tool is also provided with apparatus for formationevaluation or correlation, such as electric logging or radioactivelogging, and apparatus for detecting other well and formation parameterssuch as, conductivity, resistivity, porosity, temperature, etd. The toolsystem also incorporates a system for telemetry, such as a drillingfluid column pulser, a single conductor wireline, electromagnetic wavetransmission and receiving apparatus, or solid state recoverable memoryfor efficiently transmitting all of the well and formation data tosignal receiving equipment located at the surface while drilling is inprogress. A power supply and associated signal preparation, andtransmission electronic devices are provided within the housing.

In a MWD configuration, batteries located in the tool can be charged bya drilling fluid driven turbine generator incorporated within thesystem. Likewise the electrical current produced by the turbinegenerator will be available for other uses such as current injectioninto the formations to enhance the electromagnetic field of the target.Use of a downhole turbine driven generator will enable the availabilityof electrical power magnitudes greater than those available with:current electric wireline transmitted capabilities.

The surface system includes a receiver for receiving the serial signaland a computer which decodes and separates the composite signal intosubsignals indicative of the many measurements taken downhole. In theMWD configuration, the receiver at the surface would comprise, in part,a pressure transducer for converting pressure pulses into electricalsignals. The interface also receives a signal from a depth indicatorwhich indicates the depth of the tool in the borehole. The computeraccepts as the values of the components measured which in turn are usedto calculate azimuth and inclination of the borehole, distance to thetarget and direction to the target. Appropriate printers, plotters anddisplays, display and plot the measured and calculated value so that thedrilling of the borehole in relation to the target may be continuouslymonitored and the borehole adjusted while drilling.

In one particular aspect of the invention, the information generated bythe tool, in addition to being used for computing distance and directionto the target and borehole orientation, may also be used to determinethe path to the interception from the borehole to the target. A methodis provided which is especially useful when drilling along anapproximate east/west line through the target well. The magnetic fieldsystem data are used to calculate the gradient of the axial component ofthe magnetic field along the axis of the borehole, and to calculate theradial component of the total field orthogonal to the axis of theborehole. The axial gradient and the radial component are thensimultaneously plotted as a function of location in the borehole. As thetool passes on the north side or approaches a "North" pole, from thenorth, the total radial intensity of the magnetic field measured isreduced. When approaching a "South" pole from the north, the totalradial intensity would increase. Conversely, when the tool passes to thesouth of a target and the axial gradient indicates the presence of anorth pole, then the total radial intensity will decrease below: thenormal value. Likewise, a south pole to the north will cause an increasein radial intensity. Knowing the inclination and direction of theborehole, the drilling direction may be altered to the north or southuntil an interception with the target is achieved.

BRIEF DESCRIPTION OF THE DRAWINGS

A typical embodiment of the instant invention is illustrated in theattached drawings which are to be considered in connection with thedetailed description that follows. In the drawings, like referencenumbers designate identical or corresponding parts throughout theseveral views.

In the drawings:

FIG. 1 is a sectional view illustrating the earth's formation andshowing the sensing apparatus in a borehole being drilled adjacent to a"blowout" target well intended to be intercepted.

FIGS. 2 and 2A are expanded views of the downhole sensory apparatus ofFIG. 1.

FIG. 3 is a diagram relating to the magnetic ranging technique andrelating to the discussion associated therewith.

FIG. 4 and FIG. 5 are diagrams of the pattern of emanation of a magneticfield existing in conjunction with the cased well of FIG. 1.

FIG. 6 is a Vector diagram of the coordinate axis system defined by theset of orthogonal magnetic field sensors carried by the subsurfacesensing apparatus disposed in the open borehole.

FIG. 7 is a vector diagram relating to the development of correctionfactors for use in calculation of borehole elevation and azimuthcorrection angles.

FIG. 8 is a plot of magnetic field data taken in a typical relief wellas it approached a target.

FIG. 9 is a chart showing the magnetic response to sensed North andSouth poles.

FIG. 10 is a cross-sectional view of one embodiment of the subsurfacesensing apparatus.

FIG. 10A is a cross-sectional view of an alternative embodiment of thesubsurface sensing apparatus.

FIG. 11 is a block diagram of subsurface electronics in the subsurfacesensing package.

FIG. 12 is a block diagram of the surface electronics instrumentation.

FIG. 13 is an elevational view in section illustrating an embodiment ofthe invention incorporating an electrical system for injecting direct oralternating current into the formation.

FIG. 14 is a partial cross section through FIG. 13 illustrating thebifurcated guide and latch device for current injection.

DESCRIPTION OF THE PREFERRED EMBODIMENT A. General Theory

The general theory upon which the method and apparatus of the instantinvention is based is that generally descriptive of and applicable tomagnetic and gravity fields. The focus of this invention, however, isthe use of magnetic and gravity fields to determine the orientation andpath of the borehole with respect to the earth and the orientation ofthe tool within the borehole, and to combine this information withinformation about drillstring dynamics and with the formation evaluationmeasurements made concurrently by the conductivity, resistivity, andradioactivity or other sensors and to transmit the data while drillingto the surface for calculations, readout, and use by surface personnel.The tool also can be used to determine while drilling range anddirection to a subterranean target which exhibits or may be induced toexhibit a magnetic field.

The present invention utilizes the characteristics of the earth'smagnetic field and gravity field along with the magnetic field of atarget source to provide information from which the orientation of aborehole and the range and, direction to the target source from theborehole may be determined. The borehole orientation or correlationinformation can be combined with the concurrently taken formationevaluation measurements to completely map the formation during thedrilling and steering operations. Alternatively, where a formation hasalready been mapped, the orientation measurements provide additionaltool and borehole orientation information which can be used fordirectional drilling.

FIG. 1 and FIG. 2 illustrate one application to which the method andapparatus of the present invention may be applied, that is the drillingof a directional relief well to intercept a previously drilled well. Asshown in FIG. 1 the blowout well 10 forms a well bore 11 extending to aproducing formation 12. For the purpose of killing the blowout well arelief well 13 is drilled providing an directional wellbore 14 so as toobtain a near intercept, or a direct intercept of the blowout well inthe producing formation. Drilling fluid and other materials are thenpumped into the formation close to the blowout well. These very heavyfluids are forced into the blowout wellbore, and toward the surface bythe bottomhole pressure. At some level, the column length of heavyfluids in the blowout well bore will provide a hydrostatic pressureequal to the bottom hole blowout formation pressure and thus additionalformation fluids will no longer be forced into the blowout well bore. Atthis point the blowout well will be dead.

As shown in FIGS. 2 and 2A the downhole tool shown generally at 15 ispositioned inside a drill collar 16; composed of a nonmagnetic materialsuch as monel, which is connected to the nonrotating drillstring 23extending from the surface. Below the drill collar is connected a muleshoe sub 17 incorporating a mule shoe key 18 which provides adirectional reference A float sub 19 incorporating a check valve isconnected to the mule shoe sub and provides support for a bent sub orsteerable assembly 20. A downhole motor 21, which is a turbine motoroperated by flowing drilling fluid, extends from the bent sub andsupports a suitable drill bit 22. The downhole drill collars can alsoincorporate a turbine driver electrical generator to provide electricalpower for operation of the electronic circuitry of the tool and forsupplying electrical current for other activities, for example,injection of electrical current into the formation. In particular, anonmagnetic monel or nonmagnetic stainless steel encased mud motor 21could be utilized to minimize interference with the magnetic ranging anddirectional survey information or additional monel collars could bescrewed into the top of the mud motor to displace it vertically from themonel measurement collar. Adjustments or compensation to account for theinterference from the magnetic material in the borehole can be made insoftware, and in addition, or alternatively, insulating fiberglass drillpipe joints could be inserted to cut down on the electrical conductivityof the drillstring.

1. Borehole Orientation

The earth's gravity and magnetic fields may be used to continuouslydetermine the inclination and azimuth of the borehole. Additionally, therotation of the tool in the hole relative to a fixed coordinate systemmay be determined.

The earth's coordinate system is:

X horizontal and north, Y horizontal and east, and Z vertical and down.

The tool coordinate system is similar, but referenced to the frame ofthe device which is in turn referenced to the drilling system by thelocking keyway and pin. It is:

X' and Y' perpendicular to one another and in a plane perpendicular tothe longitudinal axis of the tool; and

Z' along the tool longitudinal axis and directed downward.

X' and Y' alignments are measured by the X and Y component magnetometersand the X and Y component accelerometers.

Thus the reading of these sensors may be used in standard geometrictransformations to provide redundant values of borehole azimuth andinclination, and tool rotation. The values are continuously computed andprovided to the drilling personnel.

In the typical case, the magnetic field reference system is used incombination with the gravity field reference system to provide boreholeand drilling system orientations. In some cases, such as when the targetis nearby only the accelerometer information is used. In any case, bothare available as a check.

1. Target Range

Large pieces of magnetic material, such as magnetized casing ordrillstring in a borehole, can create anomalies in the earth's magneticfield. An anomaly of this sort will appear as a magnetic field ofintensity H superimposed on the earth's field. The general form of theexpression for the magnetic field as a function of distance from theanomaly is given by:

    H=KM/r.sup.n                                               (1)

where K is a constant dependent upon such properties as magneticsusceptibility of the material M is the magnetic moment of the magneticbody, and n is the fall-off rate with distance, r, of the magnetic fieldintensity H of the body.

Differentiating the above expression yields of the rate of change of themagnetic field intensity with respect to radial position from the centerof the magnetic body. The derivative is: ##EQU1## and expresses a vectorquantity that may be referred to as the gradient of H, or grad H, in theradial direction. By forming the ratio of H/dH, an expression resultsinvolving only the range, r, to the magnetic body and the fall-off raten. That expression is: ##EQU2##

If two measurements are made such that ##EQU3## then upon division,##EQU4## or in the alternate, ##EQU5##

This derivation indicates that the range, r, of an observation point inspace from the magnetic body can be determined from magnetic fieldintensity taken at three or more points along a substantially straightline representing the axis of the relief well to determine the averagegradient of the magnetic field between those points.

The values of H and dH/dr for the above equations can be measured usingtwo aligned magnetic field sensors displaced a fixed distance apart. Forgreater accuracy, an average of the magnetic field intensities measuredon two magnetic sensors can be used for the value of H. The differenceΔH in the readings between two magnetic sensors divided by theseparation Δr between them yields ΔH/Δr, which is the average gradientof the magnetic intensity H over the separation and a good approximationof dH/dr. In practice, these measurements are made continuously andrepresentative points picked from the recorded curves for theappropriate component.

Referring to FIG. 3, a diagram is presented therein illustrating theforegoing discussion. In order to obtain two measurements of H and ΔH/Δrfor substitution in the above equations, it is necessary to make atleast three measurements of the magnetic field intensity.

Therefore, to obtain H₁, the magnetic field intensity at points a and bmust be measured and averaged. The separation of the magnetic sensorsdefines points a and b, with Δr being the distance therebetween. Theapproximation of dH₁ /dr is obtained by dividing the difference in themeasured field intensities at points a and b, designated H₁, by theseparation Δr. To obtain H₂, the displaced magnetic sensors are moved toa new location along the common axis, with the sensor previously atpoint a moving to point b and the sensor previously at b moving to pointc. Similar to the determination of H₁, the magnetic field intensity ismeasured at points b and c with the value of H₂ being the average of thetwo measurements. The approximation of dH₂ /dr is obtained bydetermining the difference between the intensities at points b and c,ΔH₂ and dividing that quantity by the separation, Δr. The value of r, inequation (4) above is found in FIG. 3 to be r₁ =r+3Δr/2, and the valueof r₂ =r+Δr/2. Measurements would be repeated at intervals as thesensors are advanced along a path to update and monitor the closing ofthe range. Ranging accuracy can be improved with the measurements beingmade at intervals that are closer together, approaching a continuousrecording.

By substituting the above determinations into equation (4), thefollowing equation: ##EQU6## results, which can be simplified to:##EQU7## and rewritten to express the range, r, as follows: ##EQU8##Assuming that Δr/2 is insignificant when compared to r, the equationreduces to: ##EQU9##

The range will be expressed in whatever dimensions the separation Δr ismeasured. Typically, it would be in feet or meters.

Once the range, r, is determined, the fall-off rate, n, may beascertained to indicate the character of the magnetic target. The valueof n is obtained by solving the equation: ##EQU10## or the approximationformula ##EQU11##

It is to be appreciated that the ranging technique described above canalso be carried out with a single magnetic sensor. If only one sensor isused, the measurements of magnetic field intensity must be correlatedwith the distance down the borehole (the Δr distance) at which they aretaken in order to ascertain the separation between the points at whichthe measurements are made. This can be done by suspending the sensorwith a cable that is marked to indicate its length. The separation isrequired to permit the average gradient of the magnetic field, ΔH/Δr, tobe determined.

It is to be pointed out that ranging with a single magnetic sensor cannot, because of practicalities, be as accurate as with two sensors offixed separation and known alignment with respect to one another. Mostimportant of the practical limitations on using a single sensor is theinability to be sure that the sensor is oriented the same at allmeasurement locations. It is a basic premise of the ranging techniquethat the field intensity measurements be made along a straight line andthat the magnetic field sensors not change in orientation. If not,appropriate geometric modifications must be made.

While the above theory works, in application the accuracy and confidenceis improved if the components of the magnetic fields are measured alongmutually orthogonal axes which are permanently aligned along the axis ofthe borehole.

3. Target Direction

Magnetized structures of various dimensions and configurations createmagnetic fields having a characteristic emanation pattern. For example,a magnetized elongated structure forming a magnetic dipole will havemagnetic flux lines emanating from one end to the other. However, if thestructure is sufficiently long and the point of observation is movedproximate one end, the magnetic body will appear to be one single pole,body with outwardly, radially directed flux lines extending from theelongate magnetic structure. In practice, most situations reduce tothese monopole configurations emanating from pipe collars or otherimperfections. The magnetic field characteristics can be utilizedthrough appropriate detection by magnetic field sensors, with properinterpretation of the measurements and knowledge of the earth's field,to determine direction to the magnetic body from some point in space.

One situation confronted in directional subsurface drilling involves awell casing or a length of drillstring which is the magnetic body to bedetected, as in FIG. 1. With the elongate configuration creating adipole and with the observation point in space being located at adistant point far away from the structure, the magnetic field emanatingtherefrom will appear to be a radially directed field, as illustrated inFIG. 4 and FIG. 5, with intensity given by H=KM/r². Utilizing a set ofthree magnetic sensors arranged orthogonally, the earth's magnetic fieldand the target's field can be detected and expressed as threecomponents. Since the earth's magnetic field may be determined in aregion not influenced by the target, its contribution in the readings ofthe three sensors can be subtracted out, leaving only the componentvalues of the target's magnetic field in the coordinate system definedby the orthongonal magnetic sensors. The component values can beresolved using conventional vector-analysis techniques to yield anindication of the direction to the target magnetic body.

Referring to FIG. 6, there is an illustrative diagram of a magnetictarget and the coordinate system defined by magnetic sensing apparatusadequate to serve as an example to which the theory and approach todetermining target direction can be applied. The coordinate axis systemdefined by the three orthogonal magnetic sensors has its three axesreferenced as X', Y' and Z'. The horizontal X' axis and the slantedoff-vertical Y' axis are perpendicular to the axis of the borehole whichis the Z' axis. Due to the slant (sigma) of the borehole, the coordinateaxis system formed by the orthogonal magnetic sensors is rotated aboutthe X' axis; and while having a common origin, the magnetic sensorcoordinate system and the surface coordinate system do not coincide.

The magnetic field sensors associated with the X', Y' and Z' axes willmeasure the magnetic filed intensity components of the total magneticfield (i.e. earth and target). The measured component magnetic fieldintensities of the target field will be referred to as H_(x) ', H_(y) 'and H_(z) '. The diagram of FIG. 6 will also serve as a vector diagramwith the reference designations H_(x) ', H_(y) ' and H_(z) ' indicatingrelative magnetic field components attributable to the target magneticbody.

With the magnetic sensors still a significant distance from the targetsuch that there is no contribution by the target's magnetic field to themeasured component values, the earth's magnetic field components in theX', Y' and Z' coordinate axis system can be determined. While theearth's field does have a gradient, it is so slight as to be regarded asinsignificant and its intensity treated as a constant. As the field ofthe target becomes measurable with the advancement of the magneticsensors down the offset borehole, the measured earth's field componentscan be subtracted from the total field components being detected by thesensors, thereby leaving only the components due to the target's fieldin the X', Y' and Z' coordinate system.

Knowing the components of the target field, the location of the targetwith respect to the origin of the X', Y' and Z' coordinate system can bedetermined.

A complete description of the components of the earth's magnetic field,H_(e), in the axial and radial directions can be calculated for anydepth location of the magnetic sensors in the subsurface borehole. Inorder to formulate this description, knowledge is required of the totalfield intensity , H_(T), and the dip angle φ, of the earth's magneticfield at the specific location on the earth where the borehole is to bedrilled. The total field intensity and dip angle can be obtained fromthe U.S. Navy Hydrographics Office.

It is also helpful to know the angle of inclination σ, from vertical andthe direction Θ, from magnetic north, of the various depths of interest,of the borehole. This information is obtained prior by taking magneticfield measurements with the subsurface magnetic sensing apparatus.Alternatively, a determination of borehole direction and deviation fromvertical, referred to as inclination, at various depths is obtainablethrough a simultaneous survey conducted by accelerometers in conjunctionwith the magnetic measurement. The course and direction of the boreholeis then determined. With the above information, the component values ofthe total field, H_(T), is in the X', Y', Z' coordinate axis system canbe expressed by the equations:

    H.sub.X '=H.sub.T cosφsinΘ

    H.sub.Y '=H.sub.T {sinφsinσ+cosφcosΘcosσ}

    H.sub.Z '=H.sub.T {sinφcosσ-cosφcosΘsinσ}

The predicted values of the earth's magnetic field in the X, Y, Zcoordinate system may be used to check out proper operation of themagnetic sensors. Also, deviations from the predicted values can be usedto indicate the presence of a magnetic target.

To illustrate the above equations, assume that the earth's field, H_(e),is 43,168 gammas and the dip angle is 37.6°. Further assume that theborehole direction is 35.5° and the borehole inclination is 38.9°. Fromthe above equations, with H_(T) =H_(e), the earth's field componentmeasured by the X' axis magnetic sensor is 18,877 gamma. The componentmeasured by the Y' axis sensor is 38,736 gamma, and the component alongthe Z' axis is 2,575 gamma. To check the values, they may be resolved toa resultant according to mathematic expression: H_(T) =H_(x) ² +H_(y) ²+H_(z) ². Substituting the above values yields the earth's field of43,168 gamma, as it should.

Continuing with reference to the diagram of FIG. 6, from the magneticfield intensity components H_(x) ', H_(y) ' and H_(z) ' measured by theorthogonal magnetic sensors, the azimuth correction angle Θ_(c) and theelevation correction angle σ_(c) can be determined. Assuming no rotationof the coordinate axis system about the Z' axis, the azimuth correctionangle Θ_(c) can be determined as: ##EQU12## The elevation correctionangle σ_(c), can be determined as: ##EQU13##

If rotation of the X', Y', Z' coordinate axis systems occurs, there willbe no change in H_(z) '; however, the values of H_(x) ' and H_(y) ' willbe affected. The vector diagram of FIG. 6 illustrates the followingcalculation which provides corrected values for the component valuesH_(x) ' and H_(y) '. The corrected values are used in the aboveequations for the azimuth correction value Θ_(c) and the elevationcorrection value σ_(c). In the diagram

of FIG. 7 and the following and calculations, τ represents the angle ofrotation of the coordinate axis system. From the diagram and beginningwith the expression: ##EQU14## which can be written as: ##EQU15## andsimplified to:

    H.sub.y cosτ=H.sub.y '+H.sub.x sinτ

from which can be shown that the corrected value is

    H.sub.y '=H.sub.y cosτ+H.sub.x sinτ

Further, it can be readily appreciated that:

    H.sub.x '=H.sub.y sinτ+H.sub.x cosτ

The resultant, R, in the vector diagram of FIG. 6 should not be confusedwith the range, r, determined in accordance with the ranging techniquepreviously described. The resultant, R, relates only to thedirectionality of the detected magnetic target, and its magnitude ismerely indicative of the total target field strength. The value of thefield can be calculated according to: ##EQU16##

The foregoing discussion of target direction determination has been withrespect to the detection of static magnetic fields; however, analternate approach may be used if a time varying magnetic field can beset up about the target. In order to set up a time varying magneticfield, a well casing or the like is excited with an alternating current.The field resulting from this type of excitation will, if diagramed,appear as a series of concentric rings emanating from the target source.The circular flux lines of the field will be directed in accordance withthe familiar "right hand rule. " The intensity of field produced willfall-off at a rate inversely proportional to the distance from thetarget source, i.e. H =KI/r.

A set of alternating or time varying magnetic field sensors aligned withthe previously described tool coordinate system is used to detect thetime varying magnetic field and measure direction and distance to thetarget. When placed in the time varying magnetic field described above,the component values would be used to determine the value andorientation of the tangent vectors. The directions to the target wouldbe normal to this set of vectors and the field values and gradients thusmeasured would be used to determine distance in accordance with thegradient ranging techniques described herein.

Although the above method has proven effective, it has been found thatthe correction angle can be quickly calculated using differences in theradial magnetic field over a distance. The angle of correction from thetool direction has been found to be approximated by the expression:##EQU17##

As indicated above, the tool orientation, with respect to magnetic northmay be expressed as: ##EQU18##

Thus the target direction may be expressed: ##EQU19##

A single illustration indicates the ease of the calculation. Referringto FIG. 8, there is illustrated H_(X), H_(y). At the depth of 1891 feet,H_(y) is shown to be -1300 gamma, H_(x) is -1600 gamma from theirrespective baselines and thus tan -1H_(y) /H_(x) is 312°.

The direction to the blowout is thus

    312°-180°-39°=93°

The 180° takes into account of the fact that the polarity of the targetis negative.

Similarly as to the gradient ranging techniques discussed above twomagnetic sensors in both the X and Y axes may be spaced apart onspecific distance r to provide a constant over which the H_(x) and H_(y)are measured. These sensors differences are

    H.sub.x1 -H.sub.x2 =Δx

    H.sub.y1 -H.sub.y2 =ΔY

When divided by the distance r they are expressed as: ##EQU20##

Then the above expression for target direction may be expressed as:##EQU21##

4. Path To Intersection

As noted above, the X', Y' and Z' components of the magnetic field maybe measured independently, with the X' and Y' components beingperpendicular to one another and perpendicular to the Z' components.However, instead of resolving all three components into a single vectorto give direction to the target, the X and Y components may beindependently resolved to provide a total radial vector indicative ofthe strength of the magnetic field in a plane perpendicular to thelongitudinal axis of the borehole.

Referring now to FIGS. 9, there is illustrated the response of thevarious static field magnetometer to two poles of opposite polarity suchas might be encountered as the relief well borehole passed near a stringof pipe in a blowout well. The poles might be found at successive pipeconnections. For purposes of the plots and the following discussion, thestatic field magnetometers are designed and used so, that when the axisof the magnetometer is aimed toward a north pole, such as that of theearth, the electric output of the sensor is positive.

Referring again to FIGS. 9, the earth's magnetic field has beensuperimposed upon the target field responses created by the north orsouth poles. The axial gradient response is in the same sense regardlessof position north or south of the target and identifies a north pole anda south pole. However, as shown by the plots, the total radial field tothe north of the target has been reduced in the space near the northpole and the total field to the south of the target has been increased.The opposite is true for the south pole. Thus, if a borehole were beingdrilled toward the target substantially along the east/west line ineither direction and the borehole was north of the target, the totalradial components would be less in response to the north pole than theradial component of the earth's field. If the borehole were south of thetarget containing a north pole, the total radial component would begreater than the radial component of the earth's field. The reverse ofthe above will be apparent when the pole is a south pole. Note that whenthe "gradiometer" comprised of Z₁ and Z₂ axial magnetometers is in aborehole and passing the space near a north pole, the peak response ofΔHZ with sensor polarity convention as described will occur when thesensors measuring H_(Z) 1 and H.sub. Z 2 are centered about the positionof nearest approach to the north pole and the above peak response isnegative.

The axial gradient response thus indicates the polarity of the target ata given location and a comparison of the radial response effect on theearth's field with the axial gradient response will indicate whether theborehole is- north or south of the target.

While comparison of the axial gradient With the effect of the earth'sfield on the total magnetic field is one way of determining whether theborehole is north or south of the target, a faster and simpler way is tosimply plot the total radial component as the function of depth alongwith axial gradient as the function of depth. FIG. 9 shows thisplotting. When the two curves, the one for the total radial and theother for the axial gradient are in phase, that is when both indicateeither positive or negative responses, the borehole is south of thetarget. When the total radial plot or curve is out of phase with theaxial gradient, the borehole is north of the target. If this plotting ismonitored continuously while drilling, guidance can be given to thedirectional driller to drill either north or south depending upon whichside of the target the approaching borehole is on. The driller willsimply drill in the opposite direction using the information from thegravity sensors and knowing which direction he is drilling until thecurves shift, if they were in phase until they shift out of phase or ifthey were out of phase until they shift in phase. The driller will thusknow when he crosses the north/south line and can drill back theopposite direction until an interception is made.

The just described magnetic ranging and steering device can also beconsidered a very accurate directional survey instrument. As the tool isbeing steered, the directional survey information can be simultaneouslyrecorded. If the drillstring uses a steerable assembly or double jointedassembly there will be no need to stop drilling, remove the drillstring,and run a directional survey instrument into the open hole.Consequently, much time will be saved. A steerable assembly is simplyrotated when the driller wants to drill a straight hole.

B. SURVEY SYSTEM APPARATUS

A survey system in accordance with the present invention forimplementing the above theory and techniques of logging a borehole andsurveying a subterranean target includes a subsurface tool, transmissionand receiving means, and surface instrumentation.

Referring now to FIGS. 10 and 10A, two alternative embodiments of thesubsurface system apparatus are depicted. FIG. 10 illustrates a magneticand gravity field sensing device 100 in a wireline telemetryconfiguration. FIG. 10A also depicts the magnetic and gravity fieldsensing device 100, but in addition illustrates the entire nonmagneticdrill collar or housing 105 of an MWD mud pulse telemetry configuration.Both subsurface instruments incorporate a magnetic and gravity fieldsensing device 100 having magnetic and gravity sensors and associatedcircuitry which provide accurate component type triaxial magnetometers111, 113 and accelerometers 119, 120, 121 capable of detecting minutemagnetic and gravity fields and accurately determining the orientationof the borehole. In the wireline tool configuration as depicted in FIG.10, the magnetic and gravity sensing devise can be lowered down into adrill collar similar to that marked as 105 in FIG. 10A. The tool 100would be lowered into place by the wireline 106 as will be described inmore detail below. In the MWD or mud pulse configuration as depicted byFIG. 10A, the tool 100 is built into the drill collar 105 which may beprovided with a side entry port and wireline connection similar to thatin FIG. 10A. However in the MWD or mud pulse configuration the wirelinewould be used only to obtain a readout of information which may bestored in the downhole memory in the event that the combination mudpulse and survey sub 105 becomes stuck. The arrangement of themagnetometers permits the measurement of three magnetic field componentssimultaneously at two predetermined locations within the instrument. Theaccelerometers and magnetometers provide continuous signals which aredigitized and multiplexed into a single signal stream and transmitted upthe borehole to the surface by suitable telemetry such as wireline ormud pulsing. The magnetic and gravity sensors are arranged, to standalone or to be used in combination with one or more formation evaluationor correlation devices provided as modules. For example, devicesproviding formation survey signals such as permeability, temperature,radioactivity, conductivity, resistivity, neutron, density, andultrasound may be incorporated in or with the tool to provide drillingpersonnel with desired formation parameters. Additionally, informationconcerning drillstring dynamics such as weight on bit, torque, anddrillstring harmonics can be sensed downhole and telemetered to thesurface.

Various arrangements are used to measure resistivity depending upon thetype measurement required. Direct resistivity measurements areaccomplished by passing an electric current out into the formation.Induction measurements are accomplished by driving higher frequencyalternating current through transmitter coils, thereby inducingsecondary current flow in the formation, which in turn creates fieldswhose strengths are a function of the formation conductivity, and whichare detected by receiver coils. The receiver coil measured parametersare proportional to the conductivity of the formation.

The direct resistivity measurement can also be used where the drillingmud is more conductive, or saline, in contrast to the induction orconductivity measurement which is more effective in oil based muds.Depending upon the conditions expected or encountered, the combinationtool can be provided with capabilities to measure either or both. Whereboth are used, comparing the data provides information on profiles ofthe formation and very good information on water saturation. When thisinformation is correlated with the information from the accelerometersand magnetometers of the steering and directional survey component ofthe tool, an even more precise profile of the formation emerges. Andtrue vertical formation thickness and true vertical depth (TVD) can beeasily ascertained.

With the addition of electromagnetic: transmitters and receivers to thetool having spaced receiver antennas, at known and constant distancesfrom the transmitting antennas, the formation can be logged bypropagating electromagnetic waves and measuring or inferring theresistivity from difference in phase between the two signals at thereceiving antennas. An electromagnetic wave propagation resistivitylogging tool works well with many mud types, has excellent resolution,and requires less nonconducting material than many other logging tools.These sensors could be built into a steel collar without the requirementof a non-magnetic material. The receiver component would be a dualchannel super heterodyne receiver with a highly stable phase detector.Both the transmitter and receiver are interfaced and connected to themultiplexing and digitizing apparatus for telemetry to the surface. Thephase differences which are detected downhole are telemetered to thesurface microprocessing apparatus where that information is processed bynumerical analysis techniques to infer the resistivity of the formationwhich as described above is very useful in the search for hydrocarbons.As with the various other formation evaluation techniques describedherein, this particular electromagnetic wave propagation technique canbe used both with electric wireline devices and with MWD or mud pulsetelemetry systems, and it also can be combined as desired with otherformation evaluation techniques for correlation.

Another possible module of the combination tool would comprise ahigh-energy gamma ray source and gamma ray detection device. The degreeof scattering of the gamma rays is proportional to the electron densityand hence to the bulk density of the formation, and the formationporosity can be calculated.

Yet another module to the tool would provide a high-energy neutronsource and a low-energy neutrons detection device. The neutrons reboundfrom heavy nuclei with high-energy but lost energy when they collidewith light hydrogen nuclei. The low-energy neutron detected at the toolare proportionally related to the hydrogen present in the formation, orstaged differently, are proportional to the water and hydrocarboncontained in the porosity. It is thus an excellent porosity measurementindicator.

The density and neutron porosity measurements can be done simultaneouslyand compared. Because the two measurements give different responses indifferent lithologies and to the presence of clay and gas, or water andhydrocarbon, a comparison will allow determination of the composition ofthe lithology and the formation's content of clay and gas, or water andhydrocarbon, and further will give a more accurate porosity measurement.

It may also be desirable to incorporate a module for detection of thenatural gamma radiation of the formation. For example, shales containpotassium with radioactive isotopes and thus provide a consistentmaximum. Normal sandstones and carbonates display little or noradioactivity, and micaceous or argillaceous sandstones or carbonatesfall in between. The natural gamma radiation detected therefore providea correlation to other measurements.

The above mentioned formation evaluation or survey devices can either beincorporated into the housing of the tool or individual modules orsurvey devices can ,be fitted into a separate sub or drill collarhousing and stacked to provide the desired range of information.Suitable connections would be provided to link these sensors in separatesubs to the microprocessor, multiplexer, and digitizer for telemetry tothe surface.

Signals from all of the mentioned survey devices would be digitized andmultiplexed for transmission to the surface signal processing equipmentand provisions can be made for the survey devices to receive controlsignals from the surface.

As mentioned, in the conditions usually encountered, suitable telemetrymethods would be electric wire line or mud pulsing. In someapplications, such as extreme fast drilling situations using an MWDconfiguration, mud pulse telemetry might not be sufficient because ofthe volume of data required to be transmitted for magnetic ranging,especially when magnetic ranging is combined with several formationevaluation measurements. A faster telemetry alternative to therelatively slower telemetry by mud pulse, as opposed to an electric linewould be to accumulate data in down hole memory. At timed intervals thedrilling could be momentarily stopped and an electric line run down tomake a temporary wet connection to retrieve the data. This would providenear real time read out of the data collected.

An alternative telemetry mode could utilize an electric line run alongthe side of the drill pipe which could enter through a side entry sub tomake a wet connection at the top of the combination tool.

The surface instrumentation provides data processing equipment fordecoding and manipulating the data obtained by the subsurfaceinstrumentation. A receiver and interface are provided for the decodingof the multiplexed signal and adding a depth signal. A computer isprovided in which the data are stored and processed. Processing of thedata is in accordance with predetermined programs that manipulate thedata to calculate borehole orientation and range and direction to thesubterranean target. Recording, printing, plotting, and displayequipment are provided so that measured and calculated values may bemonitored continuously and in real time.

Subsurface Field Sensing Equipment a. General

The subsurface instrument is designed to detect static and time varyingmagnetic fields as well as the earth's gravity field. To provide suchcapability, the instrument includes multiple sensors to provide a DC orstatic magnetic field sensing system, and AC or time varying fieldsensing system and a gravity field sensing system. When static magneticfields are to be detected, referred to as the passive mode of operation,the instrument's static magnetic field sensing system is utilized. Whenoperating in the active mode, as when time varying fields are to bedetected, the time varying field sensing system is activated. Thegravity and static magnetic field sensing system are is utilized in bothmodes for borehole and tool orientation.

Basically, the DC magnetic field sensing system comprises a pair oftriaxial magnetometers defining X, Y, Z coordinate systems. The X, Y, Zaxes are aligned with the X and Y axes perpendicular to the longitudinalaxis of the instrument and the Z axis lying along the longitudinal axisof the instrument. The two magnetometers are spaced apart apredetermined distance in the instrument so that the magnetic fieldcomponents along the X, Y, and Z axes may be measured and differenced sothat the gradient of each component can be calculated. Using thisinformation the range and direction to the detected target may bedetermined by the method described above.

The time varying field sensing system comprises a triaxial pick up coilmounted adjacent to and aligned with the two triaxial magnetometers, oralternatively, three orthogonal pick-up coils aligned in the instrumentin the same fashion as the two magnetometers. The time varying field isestablished by an electrode which is fixed within and through the drillpipe a distance above the sensor and which injects current into thevolume of earth surrounding the drillstring and including the target.Sufficient power for the current injection is available from a muddriven turbine generator which should provide at least in the range of100V measured peak to peak, at a few amps of current. The electrode isisolated from the drill string and the drilling assembly is insulatedfor some distance, as much as 200 feet above the electrode.Alternatively the time varying or AC field is established by injectingcurrent into the target well without the need for insulating thedrilling assembly. The target well is isolated from the return path,which may be another well or a ground electrode system isolated from thetarget well.

Excitation of the target by current produces a circular magnetic fieldaround the target. In the downhole measurement tools, the circuitryassociated with the pick-up coils tunes each pick-up coil to thepredetermined frequency of the current injected into the target. Thetime varying or AC magnetic field sensors can then be used to detect thetime varying component of this field and determine range and directionto the target.

Gravity sensing is provided by at least two accelerometers with theiraxes aligned with the magnetometers axes that is, the accelerometers arealigned perpendicular to one another and parallel with the axes of themagnetometers.

A power supply provides the required voltages for the sensors as well asthe subsequent signal preparation and transmission components. Theanalog output from each of the sensors is connected to an analogmultiplexer which in turn is connected to an analog-to-digitalconverter. The digital information is then transferred by amicroprocessor to a modem which transmits the signal to the surface on asingle conductor wireline or mud pulse telemetry system.

b. Mechanical Configuration

The primary difference between the two versions of the tool 100, arethat generally, the wireline tool as in FIG. 10 is designed to beinstalled and retrieved into a drill collar by the wireline 106, and isalso designed to utilize the wireline 116 for telemetry of informationfrom the downhole sensors to the surface. In contrast to the wirelineconfiguration, the mud pulse tool depicted in FIG. 10A provides amagnetic and gravity instrument 100 which is built-in to a nonmagneticdrill collar 105 which is installed just as any other drill collar wouldbe into the drill string with the tool 100 in place. Because the tool100 as illustrated is built-in the drill collar there would be no needfor the wireline to lower or retrieve the tool and there would be noneed for the, nose cone 104, keyway 103 or any other provision forconnection into a mule shoe sub.

Referring now to FIGS. 10 and 10A, there is shown in FIG. 10 across-sectional view of one embodiment of a subsurface field sensingapparatus, referred to as the tool 100 having a generally cylindricaland elongated configuration. The following description of the tool 100and its internal components apply generally to both telemetryconfigurations, wireline and MWD or mud pulse. The body portion of thetool comprises a tubular outer housing 102 of non-magnetic material,preferably a nonmagnetic stainless steel, having a nose cone 104 at thebottom end and the connector housing 106 at the upper end. Nose cone 104includes an adaptor 108 having threads 110 thereon which provide a meansfor attaching the nose cone 104 to housing 102. An alternate embodimentwould provide a nose cone adaptor providing for connection to additionalsubs which would be fitted into the drillstring below the magnetic andgravity survey sub. By providing such an electrical connection thevarious survey devices which have been described above could be tiedinto or interlocked with multiplexing, digitizing and telemetry systemfor transmittal of information to the surface. Alternatively the variousdevices could be built into one sub or drill collar which is providedwith mule shoe connection for interlocking connection with the housing102 of the magnetic and gravity survey tool 100. Seals 101 such as0-rings or high pressure seal rings are provided for sealing the toolagainst the environment in the borehole. A keyway 103 is provided on thebody 102 of the tool which mates with the key way or pin in theorienting sub or collar 17 in the drillstring 23 which fixes the tool inrelation to the drillstring. This is shown in FIGS. 2, 2A, and 10. Thetool is held in a non-magnetic collar 16 above the pin by theappropriate length of spacer.

Enclosed within the outer housing 102 of both embodiments of the toolare the electronics for the tool 100. The various sensing devices andassociated circuit boards are carried on a frame 112 that fits withinand extends for substantially the entire length of the housing 102. Whenthe survey is performed in connection with formation evaluation toolssuch as those for measuring resistivity, conductivity, or radioactivitythis frame is inserted in a housing designed to hold all tools. Twotriaxial magnetometers, 111 and 113, are mounted on either end of theframe 112 to provide maximum separation. Also mounted to the lower endof the frame 112 are the triaxial alternating magnetic field pick-upcoils 115. Three accelerometers 119, 120 and 121 are mounted centrallyon the frame.

Referring now only to FIG. 10A there is depicted an MWD or mud pulsedtelemetry embodiment of the combination tool. FIG. 10A depicts anonmagnetic sub or drill collar 105 with threads 107 at both ends of thedrill collar for threaded connection into the drillstring. The arrows109 depicted in FIG. 10A which point in a general downward directionillustrate the direction of mud or fluid flow down through thedrillstring. The mud pumps on the surface pump the mud down through thedrillstring into the upper end 118 of the survey sub 105, and past themud pulser 127. The mud pulser 127 comprises a valve seat 129 and avalve 128 which is actuated in response to signals from the downholesurvey microprocessor apparatus. Actuation of the valve 128 to seatagainst the valve seat 129 closes the fluid or mud passage which sends apressure signal through the fluid column to the surface. In this mannerthe information collected by the downhole sensors described above can bemultiplexed and digitized and transmitted to the surface by mud pulsetelemetry. As previously described it is also possible to provide a mudpulse receiving device downhole which can receive mud pulsed signalsfrom the surface in order that signals can be sent from the surface downto the steering, sensing and survey apparatus.

Referring to FIG. 10A and the fluid flow of the drilling mud, there isdepicted in the turbine 135 which is powered by the fluid flow throughthe drillstring. The turbine 135 is connected by a shaft 136 to agenerator 137 which provides downhole power. Power from the generatorcan be used to charge or recharge downhole batteries if the tool is soequipped and is also available for powering the steering sensing andsurvey apparatus, and is available for the downhole current injectionfor formation evaluation.

As the fluid 109 flows through the turbine it next passes between thevoid spaces provided between the inner wall of the drill collar 105 andthe surface of the outer housing 102 which encloses the downholemicroprocessor, multiplexer, digitizer, and magnetic and gravitationalsurvey devices. FIG. 10A as illustrated, the tool 100 is mounted withinthe drill collar 105. Each end of the tool 100 is positioned within theinterior of the drill collar 105 and securely held by mountingcomponents 133 which are located at the extremities of the cylindricalhousing 102. A variety of configurations are possible for thepositioners 133, important considerations are that the connectorsprovide a secure and immovable support for the tool 100 within the drillcollar 105 which also allows for flow of the drilling fluid or mud pastthe tool 100 through the drill stem down toward the bottom of the hole.As will be described below this fluid can be used to power a mud motorto turn a drilling bit. Another consideration for the design of the tool100 and its housing 102 and connectors 103 to the drill collar 105 isthat the annular space between the housing 102 and the interior wall ofthe drill collar 105 should allow for sufficient flow of drilling mud sothat there is no appreciable pressure changes as the drilling fluidpasses by the tool 100.

The downhole combination tool of FIG. 10A, is provided with a second sub139 which is threadedly connected by external threads 157 on the sub orcollar 139 which threadedly connect to internal threads 158 at the lowerend of the sub or collar 105. The second sub 139 depicted in FIG. 10A isused to house other types of formation sensors as desired as shown at141. Alternatively instead of the arrangement shown with two substhreadedly interconnected the additional sensors 141 could all be builtinto a single collar 105. The advantage to building everything into asingle unit would be that there would be less likelihood for failure ofthe connection of the sensing devises to the telemetry apparatus, sincethe connection would be made during the manufacture of the unit, andwould not be susceptible to problems which might occur in the field.Advantages to providing separate components for separate sensors, wouldbe that tool units could be put together as modules to provide surveyand formation evaluation capabilities as desired in a particular case.The threads 107 shown at the bottom of FIG. 10A are utilized to connectthe tool to the bottom part of the drillstring, which could include avariety of other evaluation subs, such as a bent sub or steerable sub,or to a drillbit in the case of rotational drilling, or to a downholemud motor which would be turned by the drilling fluid in a mannersimilar to the above described turbine, except that the downhole mudmotor would be used to turn a drilling bit.

The mechanical positioning of each of the magnetometers andaccelerometers is critical, not only with respect to the outer housing102 but also to each other. An X, Y, and Z reference coordinate systemis set up with respect to the central axis of the tool, the Z axis beingalong the axis of the tool 100 with the X and Y axes being perpendicularto the central axis of the tool. The Z axis sensors of each of themagnetic sensing devices magnetometers 111 and 113, and pick-up coils115, should be precisely aligned, or alternatively, correction formisalignment may be accomplished in software.

As shown in FIG. 10 and schematically in FIG. 11, electrical power maybe supplied through the shielded power supply through a cable 106 whichis connected to the rear connector housing. The cable 106 may also carrythe signals generated by the sensing devices back to the surface and canbe used to install and retrieve the tool into or from the drillstring.Power for the subsurface instrumentation and current injection may bealternatively provided by a mud turbine 135 driven generator 137 in aMWD configuration as in FIG. 10A. The measurements may be alternativelytransmitted to the surface by a mud pulse telemetry system or stored inthe instrument for periodic retrieval or by the wireline.

The mud pulse telemetry system of the tool as depicted in FIG. 10A couldadditionally be provided with capability for connection to a wireline inthe event that the tool becomes stuck. In this case a wireline could berun down through the drill pipe and to retrieve the information storedin downhole memory. This information is very valuable and costly toobtain and such provision for obtaining even if the tool becomes lostwould be very valuable.

The wireline configuration as depicted in FIG. 10 could also be providedwith downhole memory storage for information in the event that someproblem developed with the wireline which prevented telemetry ofinformation to the surface, but which did not prevent retrieval of thetool 100 by the wireline so that the information could be read out oncethe tool was pulled back to the surface. A temperature probe and a powersupply monitor are used to provide information concerning the conditionand stability of the tool performance. The analog multiplexer receivesall signals and feeds them in serial form to the analog to digitalconverter. All are under the control of the microprocessor.

The configuration and size of the tool 100 is designed so that it willfit in a non-magnetic collar 16 in the drill string and allow asufficient flow of drilling fluid to pass around it and on to theturbine 21 and drilling bit 22 for efficient drilling of the formation.FIGS. 2, 2A.

Referring to FIGS. 13 and 14 an embodiment of this invention isdisclosed generally as 130 which achieves transfer of electrical currentfrom the power and control conductor located within the drill string 23into the formation being drilled. An electrical conductor 134 extendsthrough the drill string along with the wireline 106 which is utilizedto install and retrieve the subsurface formation data tool of FIG. 10.At the lower end of conductor 134 is provided an elongated guide andlatch device 138 which projects outwardly to a location along the innerwall surface of the drill string. Refer to FIGS. 13 and 14. The guideand latch device 138 is of bifurcated form defining a pair of dependingguide legs 140 and 142 and forming a pin latch receptacle 144 at thediverging juncture thereof. Refer to FIG. 14. The depending guide legsare of curved or spiral form such that at least one of them will comeinto guided relation with an electrical conducting pin 146 as the guideand latch device is moved downwardly along with the wireline 106 andtool 100. The electrical conducting pin 146 as the guide and latchdevice is supported by an insulating and supporting element orcomposition 148. An elongated leaf spring member 150 extends from theconducting pin 146 and has a central portion curving outwardly towardthe formation. A lower electrically insulating connector 152 connectsthe lower end of the feeler spring to the outer wall of the drillstring. A formation contactor 154 is secured to the central portion ofthe feeler spring and engages the formation with sufficient force topenetrate through the drilling fluid cake and establish efficientelectrically transmitting contact with the wall of the well bore.Electrical current, typically time varying current will be transferredfrom the conductor 134 within the drill string through the wall of thedrill string and into the formation for development of a time varyingelectromagnetic field. A centralizing leaf spring 156 is positionedopposite the leaf or feeler spring 150 and junctions to contact the wellbore and introduce a centralizing force to prevent misorientation of thedrill string.

c. Subsurface Electronics

The sensors and associated electronics provide a conditioned digitizedmultiplexed signal capable of being transmitted up the borehole to thesurface. The two triaxial magnetometers used may be models 7003xx asmanufactured by Tensor, Inc. The two accelerometers may be SunstrandModel Number 979-0150 as manufactured by Sunstrand Data Control.Additional description and operation of flux-gate magnetometers ingeneral are given in U.S. Pat. No. 4,072,200 FIGS. 12-15, and column 19line 50 to column 23, line 18. The triaxial magnetometers have threemutually orthogonal sensing elements as described. Further, descriptionand operation of accelerometers in general are given in U.S. Pat. Nos.3,791,043 and 4,083,117.

The time varying magnetic field sensors along each axis comprise a coilin parallel with a tuning capacitor as described at column 23, line19-37 in conjunction with FIG. 16 of U.S. Pat. No. 4,072,200 which isherein incorporated by reference. Each of the sensing devices producesan analog signal representative of the magnitude of the particularvariable measured for recording, readout, or computation, including:

X₁ =Static Magnetic Field Intensity of Magnetometer 111 along X axis.

Y₁ =Static Magnetic Field Intensity of Magnetometer 111 along Y axis.

Z₁ =Static Magnetic Field Intensity of Magnetometer 111 along Z axis.

X₂ =Static Magnetic Field Intensity of Magnetometer 113 along X axis.

Y₂ =Static Magnetic Field Intensity of Magnetometer 113 along Y axis.

Z₂ =Static Magnetic Field Intensity of Magnetometer 113 along Z axis.

ALT X=Alternating Magnetic Field Intensity of Triaxial Coil 117 along Xaxis.

ALT Y=Alternating Magnetic Field Intensity of Triaxial Coil 117 along Yaxis.

ALT Z=Alternating Magnetic Field Intensity of Triaxial Coil 117 along Zaxis.

GX=Accelerometer reading along X axis from accelerometer 119.

GZ=Accelerometer reading along Z axis from accelerometer 120.

GY=Accelerometer reading along Y axis from accelerometer 121.

TEMP=reading from temperature sensor.

PWR power Supply Voltage.

2. SURFACE EQUIPMENT

equipment may best be explained in conjunction with FIG. 12 which is ablock diagram of the surface system. The serialized signal from downholeis received by a second modem which decodes the signal for use by thecomputer. Additionally, a transducer identified as a depth wheel encodertransmits a digitized signal indicating the depth of the tool in theborehole. The depth signal may be easily generated by a wheel over whichthe wireline passes before entering the borehole. The apparatus isfitted with a light source and detector which transmits a pulse to thecomputer with each wheel revolution. The computer then transforms therevolutions to depth.

All of the decoded signals are passed on to a computer which isprogrammed according to the theory section above to calculate thefollowing values using the methods previously described:

TOTAL₁ =Total magnetic field intensity by resolution of components X₁,Y₁ and Z₁.

TOTAL₂ =Total magnetic field intensity by resolution of components X₂,Y₂ and Z₂.

RADIAL=Total magnetic field intensity in a plane orthogonal to the Zaxes by resolution of X₁ and Y₁.

GRAD X=X¹ -X² /separation

GRAD Y=Y¹ -Y² /separation

GRAD Z=Z¹ -Z² /separation

DEPTH=Wireline position.

AZIMUTH=Deviation of borehole from north

INCLINE=Inclination of borehole from vertical

TF=Orientation of tool face with respect to the high side of the hole.

The data received and the calculated data may be output in severalfashions and/or stored by magnetic storage as desired. A COMPAQ®Portable II computer has been found to be quite useful and versatile fordata processing and many equivalents exist. Generally, even after allthe signal conditioning, the magnetic data requires expertinterpretation while the directional data-AZIMUTH, INCLINE and TF areimmediately useful to the directional drilling supervisor. Therefore thedata is made available on separate outputs. The sensed magnetic data maybe continuously plotted as well as recorded on magnetic disc or tape forfuture processing. The instantaneous direction and distance to thetarget may also be displayed or printed. Additionally, any measured orcalculated value may be displayed on a real time or continuously updateddisplay on the computer monitor. The magnetic data requires expertinterpretation due to the many forms of magnetic field anomaliesencountered but the tool and surface equipment provide opportunities tocontinuously monitor the progress of the borehole as it nears thetarget.

C. SURVEY SYSTEM OPERATION

In operation, the downhole tool may be connected to a wireline which ispassed over the depth measurement wheel. The tool is then lowered downthrough the drill string where it seats in the non-magnetic drillcollar. Data being received and processed by the tool is thentransmitted to surface data receiving and processing equipment by singleor multiple wireline electric conductors. Alternatively the downholeinstrumentation is housed in a specially constructed Monel MeasurementWhile Drilling (MWD) drill collar. The drill collar is then made up inthe drill string and positioned close to the downhole mud motor whichturns the drill bit. Power is supplied by internal batteries, aconnected wireline, or from an internal mud turbine generator. Measuredinformation is telemetered to the surface by hydraulic mud pulsetransmission, or a connected electric wireline either inside or outsidethe drill string, or by periodically connecting an electric wirelineinto a downhole memory to receive data stored there. An alternative MWDtype tool would use the mud pulse telemetry or memory storage system.

The subsurface tool travels with the drill string down the boreholebeing drilled and the survey is made. Based upon the data provided bythe subsurface instrument,.the course of the borehole is controlled. Thedirectional drilling is altered until the subsurface field sensingapparatus determine that the borehole is aligned in the directiondesired, toward a target casing in a blowout case or away from otherwells in the case of multiple wells drilled from a single platform. Ifthe passive mode is selected, the subsurface sensing apparatus willsimply sense the magnetic field of the target superimposed upon theearth's magnetic field.

In order to orient the apparatus with respect to the surfacegeographical coordinates, it can be useful to know the field intensity,the direction with respect to magnetic north, and dip angle of theearth's field. All of these will be unique values depending upon theexact location on the earth's surface where the drilling is to takeplace. Much of this information is provided by surveys that are readilyavailable in the industry.

As the instrument is being lowered, measurements of the magnetic fieldintensity components are made. The surface instrumentation accepts themeasurements and supplies them to the computer which organizes andanalyzes the data. In the electric wireline configuration, when the toolbecomes engaged in the non-magnetic drill collar, the data iscontinuously output and transmitted up the hole to the surface equipmentas noted previously. In the MWD configuration, the data is continuouslyoutput into the hydraulic mud pulse telemetry system and transmitted upthe hole to surface equipment as noted previously. Alternately the datais stored into a memory bank where it can be retrieved by a wetconnected electric wireline conveyed tool designed to accept theinformation dumped by the downhole memory bank, or the memory bank canbe dumped when the drill collar is retrieved back at the surface afterall of the drill pipe is pulled out of the hole.

Although no techniques have been described in detail for carrying outthe calculations for target range, target direction and toolorientation, anyone skilled in the computer art can program a computerto solve the equations provided herein and to apply the technique ofvector analysis with the required data. Although the calculations may becarried out by any calculator, as noted above, the COMPAQ computer orequivalent may be used.

The foregoing description of the invention has been directed toparticular preferred embodiments of the present invention for purposesof explanation and illustration. It will be apparent however, to thoseskilled in this art, that many modifications and changes in theapparatus and method may be made without departing from the scope andspirit of the invention. It is therefore intended that the followingclaims cover all equivalent modifications and variations as fall withinthe scope of the invention as defined by the claims.

What is claimed is:
 1. A survey system used in borehole drillingapplications to determine range and direction to a ferromagnetic body,comprising:(a) a downhole tool comprising a non-magnetic tool housingwhich defines a longitudinal axis and is positionable at a locationwithin a borehole; (b) magnetic field sensing means mounted in saidhousing for generating magnetic field data, said magnetic field sensingmeans comprising at least one pair of triaxial static magnetic fieldsensors spaced apart a predetermined distance along said longitudinalaxis of said tool housing for measuring static magnetic fields; and (c)processing means operably connected to said magnetic field sensing meansfor determining magnetic field gradient information from said magneticfield data, said processing means further determining the direction tosaid ferromagnetic body using said magnetic field data and said magneticfield gradient information and the range to said ferromagnetic bodyusing said magnetic field data and said magnetic field gradientinformation.
 2. The survey system of claim 1, wherein said magneticfield sensing means further comprises triaxial time varying magneticfield sensors mounted in said tool housing for measuring time-varyingmagnetic fields.
 3. The survey system of claim 2 further comprisinggravity field sensing means mounted in said housing for generatinggravity field data.
 4. The survey system of claim 3, wherein saidprocessing means is further operably connected to said gravity fieldsensing means for determining borehole azimuth and inclination usingsaid gravity field data and said magnetic field data.
 5. The surveysystem of claim 4 wherein said at least one pair of triaxial staticmagnetic field sensors comprise triaxial magnetometers arranged formeasurement of magnetic field strength along orthogonal x, y and z axes,said x and y axes being disposed in normal relation and perpendicular tothe central axis of said housing and said z axis being coincident withthe central axis of said housing, said triaxial magnetometer providingelectrical signal outputs representing static magnetic fieldmeasurements.
 6. The survey system of claim 5 wherein said triaxial timevarying magnetic field sensor comprise at least one triaxial ACmagnetometer arranged for measurement of varying magnetic field strengthalong orthogonal x, y and z axes, said x and y axes being disposed innormal relation and perpendicular to the central axis of said housingand said z axis being coincident with the central axis of said housing,said at least one triaxial AC magnetometer providing an electricalsignal output representing time varying magnetic field measurements. 7.The survey system of claim 6 wherein said gravity field sensing meanscomprises at least one triaxial accelerometer for measurement of thecomponents of the earth's gravity field along orthogonal x, y and zaxes, said x and y axes being disposed in normal relation andperpendicular to the central axis of said housing and said z axis beingcoincident with the central axis of said housing, said gravity fieldsensing means providing electrical signal outputs representing gravityfield measurements.
 8. The survey system as recited in claim 7,including means for selectively injecting electric current into an earthformation transversed by said borehole for enhancing the time varyingmagnetic field of said subterranean ferromagnetic body.
 9. The surveysystem as recited in claim 7, including a data storage system withinsaid tool housing for receiving and storing data received from saidsurveying means and wherein said processing means is capable ofselectively accessing said data storage system.
 10. The survey system asrecited in claim 9 further comprising a means for communicating datawhich comprises an electrically conductive cable extending along saidborehole from the surface to said downhole tool and operativelyconnecting said downhole tool to said processing means to enablereal-time processing of data received from said surveying means of saiddownhole tool.
 11. The survey system as recited in claim 10, whereinsaid means for communicating data comprises a fluid pulse signaltransmitting system for translating said position-indicating data intofluid pulses, transmitting said fluid pulses through the drilling fluidin said borehole to the surface and translating said fluid pulses backinto said position-indicating data for processing by said processingmeans.
 12. The survey system of claim 11 wherein said downhole toolfurther comprises a signal multiplexing means to multiplex the signalsgenerated by said magnetic and gravity field sensing means into aserialized signals, said signal multiplexing means connected to a signaldigitizing means for digitizing said serialized signals prior totransmission, and said downhole tool further comprising microprocessorcontrol means connected to said signal multiplexing and digitizing meansand further connected to a tool housing receiving means, saidmicroprocessor control means processing signals transmitted from thesurface down a fluid column within said drill string to introduceprogrammable variables into said signal multiplexing means and saidsignal digitizing means to modify the multiplexing and digitizing ofsaid signals generated by said magnetic and gravity field sensing means.13. The survey system as recited in claim 12 wherein said microprocessorcontrol means is further programmed to receive said signals generated bysaid magnetic and gravity field sensing means and process them fortransmission to the surface while drilling is in progress.
 14. Thesurvey system as recited in claim 13 wherein said microprocessor controlmeans accepts said signals from said magnetic and gravity field sensingmeans and serializes each individual signal so that a single compositesignal may be transmitted to the surface on a single conductor.
 15. Thesurvey system of claim 14, further comprising a computer means locatedat the surface for receiving said signal composite signal, said computerincluding a display means for current visual display of downholeinformation from said single composite signal and a printer/plotter forpermanent display of downhole information.
 16. The survey systemaccording to claim 3 further comprising means, mounted within said toolhosing, for measuring formation characteristics to provide formationevaluation of the strata through which said tool is progressing.
 17. Thesurvey system as recited in claim 16, wherein said means for measuringformation characteristics includes a gamma ray source and gamma raydetection means mounted within said tool housing, said gamma ray sourceand gamma ray detection means being operatively connected to saidprocessing means to provide formation porosity information.
 18. Thesurvey system as recited in claim 17, wherein said means for measuringformation characteristics includes means for measuring electricalresistivity, mounted within said tool housing, said electricalresistivity measuring means being operatively connected to saidprocessing means to provide an electric log of a subterranean earthformation intersected by a well bore.
 19. The survey system as recitedin claim 18, wherein said means for measuring the characteristics of theformation being drilled provides a log of said formation and furtherincludes means for measuring drillstring dynamics including torque,weight on bit, and harmonics.
 20. The survey system recited in claim 19wherein said means for measuring the characteristics of a formationbeing drilled further comprises an electrical resistivity measuringmeans for measuring the electrical resistivity of said formation andproviding output signals reflecting formation density.
 21. The surveysystem as recited in claim 20, wherein said means for measuring thecharacteristics of the formation being drilled further compriseselectrical conductivity measuring means, temperature measuring means,neutron and density logging means and electromagnetic wave loggingmeans.
 22. The survey system as recited in claim 21 wherein said meansfor measuring the characteristics of the formation being drilled furthercomprises radioactive logging means for conducting a radio active log ofsaid formation intersected by said borehole.
 23. A method of directionalsubsurface drilling of a borehole to intersect a subterraneanferromagnetic target, comprising the steps of:(a) measuring componentsof a total magnetic field along orthogonal axes at any location in saida borehole sufficiently proximate to said target to detect a magneticfield of said target superimposed upon the earth's field using at leasttwo triaxial static magnetic field sensors spaced apart a predetermineddistance; (b) determining magnetic field gradient information from saidcomponents of said total magnetic field measured by said at least twotriaxial static magnetic field sensors and said predetermined distance;(c) determining target direction using said components of said totalmagnetic field and said calculated magnetic field gradient information;(d) determining target range using said components of said totalmagnetic field and said calculated magnetic field gradient information;and (e) orienting borehole trajectory dependent upon said direction andrange determination so that said borehole will intercept said target.24. The method according to claim 23 further comprising the step ofsuccessively measuring said components of said total magnetic fieldusing said at least two triaxial static magnetic field sensors spacedapart a predetermined distance.
 25. The method according to claim 24further comprising the step of successively determining said magneticfield gradient information from said successively measured components ofsaid total magnetic field.
 26. The method according to claim 25 furthercomprising the step of successively determining target range anddirection from said successively calculated magnetic field gradientinformation.
 27. The method according to claim 26 further comprising thestep of projecting an expected target trajectory and correcting therelief well drilling plan dependent upon said successive target rangeand direction determinations so that said borehole will intercept saidtarget.
 28. The method according to claim 27 further comprising the stepof measuring selected characteristics of a formation being drilled toprovide formation evaluation of the strata through which said boreholeis progressing.
 29. The method of claim 1 wherein said boreholeorientation with respect to said target is determined by the directionand range from said borehole to said target.
 30. The method of claim 29wherein said borehole orientation is further determined with respect tosaid barge as north and south.
 31. The method of claim 30 wherein saidorientation of said borehole either north or south of said target isdetermined by:(a) continuously plotting a gradient of said totalmagnetic field along an axis of said borehole; (b) continuously plottinga total radial component of said total magnetic field in a planeperpendicular to said borehole axis; (c) comparing said gradient andradial plots to determine whether an existing combination of north poleand reduced radial field or south pole and increased radial fieldindicates that said target is in a southernly direction or whether thecombination of north pole and increased field or south pole anddecreased field indicates that said target is in a northerly direction.32. The method of claim 27 further comprising transferring electricalcurrent from a location within a drill string into said formation beingdrilled at a depth location near a drill bit for development of anelectromagnetic field in said formation and for other data collectionactivities.
 33. The method of claim 32, wherein said electric current isan alternating electrical current for development of an alternatingelectromagnetic field.